1. Field of the Invention
This invention relates to cleaning of fire-heated tubes. Fire-heated tubes provide indirect heating of a process stream from a fired furnace or fire box. These tubes are analogous to heat exchangers in that both involve indirect heat transfer from one medium to another. However, fire-heated tubes are not considered heat-exchanger tubes in the art. Heat exchangers are heat recovery systems primarily to transfer heat energy from one portion of a process to that of another, whereas fired heater tubes are process stream heaters that transfer heat only into the process, rather than from one portion to that of another in the process.
2. Prior Art
Cleaning interior surfaces of heat-exchanger tubes, even when in operation, is fairly well known. This is disclosed in the following U.S. Pat. Nos.: 3,946,455(1976); 4,297,147(1981); 4,343,702(1982); 4,482,392(1984); 4,579,596(1986); and 4,583,585(1986).
Cleaning exterior surfaces by a process commonly referred to as sand-blasting or grit-blasting is well known. In Industrial Maintenance Painting (Third Edition), published by National Association of Corrosion Engineers (1967) on page 14 and the following: Abrasive Blast Cleaning is disclosed as a very economical method for surface preparation.
Sand-blast cleaning is disclosed to be useful for tubular-type bundles, tower trays, pump casings and many other refinery items. In Petroleum Processing Handbook, edited by William F. Bland and Robert L. Davidson, published by McGraw-Hill Book Company, 1967, sandblast cleaning of the outside of tubular bundles is disclosed. However, there is no discussion for such cleaning while the tubular bundles are in service.
Fire heaters, such as sold by Petro-Chem Development Company, Inc., and Foster Wheeler, involve vertically oriented heat exchange conduits along generally cylindrical walls surrounding one or more burners. The burners directly impinge upon these conduits. Decomposition products of combustion form scaly, sulfur-containing coatings. These coatings are undesirable, because with time these coating give rise to an insulative layer that prevents the transfer of heat from the flame to materials being transported within the heat-exchanger tubes. Materials that are heated in such tubes include water and, more commonly, hydrocarbon-containing feedstreams. When the material within these heat-exchangers are hydrocarbon-containing material, they are often referred to as process heaters.
While in service, exterior surfaces of fired heat-exchanger tubes are at a temperature as high as 1500.degree. F. (816.degree. C.). The abrading agent must be at an appropriate impact velocity in order to insure that any scale formation or deposits of decomposition products are removable without damaging either the heater tubes or any of the other surfaces likely to be impacted by such abrading agent.
It is an object of this invention to define the method steps appropriate to cleaning fire-heated tubes while in operation.